In the past it has been customary to form extrusion dies for forming thin-walled honeycombed structures from a solid die body by saw-cutting discharge slots in the outlet face of the die body, and drilling rather lengthly feed holes into the inlet face of the die body so as to communicate with such discharge slots, in a manner shown in U.S. Pat. No. 3,790,654 to Bagley. As further shown in the Bagley patent, the feed holes may communicate with each intersecting slot or every other intersection, as desired, however, in both cases the feed holes extend a substantial distance through a unitary die body.
In order for the extruded material to coalesce within the discharge slots into a unitary grid or matrix prior to being extruded from the outlet face of the die, it is necessary that the slots either be of sufficient length so that the extruded material will have time to flow transversely within slots to knit into a unitary grid prior to being longitudinally discharges from the outlet face of the die, or else additional feed holes must be provided in communication with the slot gridwork in order to reduce the amount of transverse flow required to provide such a unitary cellular matrix therewithin and prior to discharge from the die face.
However, since the ceramic material extruded through such dies is rather abrasive, it is necessary to utilize a high quality steel which gives extended life and which is resistant to wear caused by such abrasion. Accordingly, it is extremely difficult to machine such steel into a gridwork of slots and still maintain the necessary tolerances required to produce a honeycomb structure having a cellular density up to 300 cells per square inch with wall thicknesses of 0.01 inches or less. Therefore, the depth of the slots which can reasonably be formed while maintaining the necessary tolerances is limited to about 0.2 inches, depending upon the material of the die utilized.
Further, it has been extremely difficult to drill the rather extended feed holes through the unitary die body so that they perfectly register and communicate with the intersections of the discharge slots, not only due to the high quality wear resistance steel which must be utilized, but also due to the extended length that is required. The length of the feed holes, of course, is governed by the thickness of the unitary die body, and the thickness of the unitary die body is dictated by the strength of the die required in order to withstand the high extrusion pressures required, which may reach as high as 3000-3500 psi. In addition, it will be recognized that the diameter of the holes which may be utilized is also severely limited by the structural strength requirements of the die, for if the diameter is increased extensively to provide material to a greater area of the discharge slot gridwork, the die would become an extremely weak cellular construction which could not withstand the required extrusion pressures. Therefore, the size of the feed holes as measured by their diameter, was severely limited by the structural strength requirements of the die, which in turn only permitted the use of rather small diameter feed holes resulting in excessive pressure drops in the flow and necessitating unduly large operating pressures.
Whereas the feed holes of the die of Bagley substantially overlapped the intersections of the discharge slots and terminated in abuttment with portions of the die pins between such slots, U.S. Pat. No. 3,846,197 to Wiley suggests the use of a transition or distributor area at the lower end of the feed holes for directing the flow in a pattern similar to the intersection of the discharge slots. That is, each feed hole is provided with its own distributor zone which functions to funnel the extrudable material into the slot opening at the intersections thereof rather than into abuttment with portions of the pins adjacent the slots, resulting in improved transverse flow of the extruded material. However, the die of Wiley is plagued with the same limitations as the die of Bagley, in that the maximum hole size is severely limited by structural and strength requirements of the die, and accordingly pressure drop in the restricted feed holes is a major consideration requiring fairly large operating pressures.
Accordingly, the present invention has overcome the problems of limited feed hole size, high pressure drop and loss of die strength, by providing a completely unique manner of forming an extrusion die with a plurality of relatively large diameter feed passageways which may be of extended length, each feeding a plurality of smaller diameter passageways or feed holes of limited length which directly communicate with intersections of a slotted discharge gridwork.